The present invention relates to an equalizer and a reproduction signal processing device, and more particularly to a technique for performing a waveform equalization on a signal with non-linearity.
In a reproduction signal processing device for reproducing information recorded on a recording medium such as an optical disk, vertical asymmetry may appear in the reproduction signal read out from the medium. With an optical disk, for example, a row of pits on the optical disk is irradiated with laser light, and the intensity of the reflected light is read out as a reproduction signal. Due to variations in various conditions such as the power of light used for mastering an optical disk, the size and/or the shape of a pit on the surface of the optical disk may vary, thereby also varying the intensity of the reflected light, thus resulting in a reproduction signal with vertical asymmetry.
FIG. 8 is a schematic diagram illustrating asymmetry resulting from variations in the pit width. Pits along a row (1) have a width that is smaller than that of pits along a normal row (2). Therefore, the row (1) has a longer non-pit pattern than the normal row (2), and thus the reproduction signal from the row (1) has a higher intensity than that from the normal row (2). On the other hand, pits along a row (3) have a width that is larger than that of pits along the normal row (2). Thus, the row (3) has a shorter non-pit pattern than the normal row (2), and thus the reproduction signal from the row (3) has a lower intensity than that from the normal row (2).
FIG. 9 illustrates signal level samples obtained through A/D conversion of a reproduction signal with vertical asymmetry. With a reproduction signal without vertical asymmetry, the signal levels are distributed in the vicinity of the ideal values, which are indicated by arrows in the figure. However, signal level samples obtained through A/D conversion of a reproduction signal with vertical asymmetry are distributed asymmetrically, as illustrated in FIG. 9, where the amplitude in the upward direction (an amplitude for positive values) with respect to the zero level (the horizontal axis labeled “0”), which is represented by an arrow labeled A1 (or A2 ), is not equal to the corresponding amplitude in the downward direction (an amplitude for negative values), which is represented by an arrow labeled A1′ (or A2′).
On the other hand, a reproduction signal processing device performs a waveform equalization on a digital signal with an equalizer that typically uses an FIR (Finite Impulse Response) filter. In the waveform equalization, a linear operation called “convolution”, as shown in Expression (1) below, is performed. Note that in Expression (1), xi is an ith tap signal, ci is a tap coefficient corresponding to the tap signal xi, N is the number of taps of the FIR filter, and y is the waveform-equalized signal.
                    y        =                              ∑                          i              =              1                        N                    ⁢                                          ⁢                                    x              i                        ⁢                          c              i                                                          (        1        )            
By performing a waveform equalization using convolution, a digital signal is corrected to be closer to the ideal level of either a positive or negative value. However, when a signal with non-linearity as shown in FIG. 9 is convoluted, the results may be diverged as illustrated in FIG. 10.
A technique relating to an equalizer for performing a waveform equalization on a digital signal with non-linearity is disclosed in Japanese Laid-Open Patent Publication No. 09-153257, for example.
FIG. 11 is a schematic diagram illustrating an equalizer 500 disclosed in this publication. In the equalizer 500, one of tap coefficients cip and cin is selected in each coefficient unit 502-i (i=0 to n) based on the sign of the value of a tap signal 511-i. Then, the selected tap coefficient is used in the multiplication at a multiplier 507. In this way, the non-linearity of the input signal can be compensated for.
A group of tap signals (“tap signal group”) for the equalizer as a whole can be represented by a vector X=(x1, x2, . . . , xN), and a group of tap coefficients (“tap coefficient group”) for the equalizer as a whole can be represented by a vector C=(c1, C2, . . . , cN). Then, Expression (1) can be expressed as “y=X·C”. Herein, as the vector representing a tap coefficient group, a vector Cp=(c1p, c2p, . . . , cNp) can be used to obtain “y=X·Cp” when the tap signal is positive, whereas a vector Cn=(c1n, c2n, . . . , cNn) can be used to obtain “y=X·Cn” when the tap signal is negative. In this way, the vertical asymmetry of the input signal can be compensated for.
However, in the equalizer 500, a tap coefficient is selected by each coefficient unit 502-i based on the sign of the value of the tap signal 511-i that is received by the coefficient unit 502-i. Thus, the elements of the vector Cp and those of the vector Cn coexist in the vector C representing the tap coefficient group of the equalizer as a whole. Then, it is not possible to select the vector Cp or Cn representing a tap coefficient group based on the sign of the value of the tap signal, thereby failing to perform an intended convolution operation. Thus, with the equalizer 500, it is difficult to accurately compensate for the non-linearity of the input signal.